Advice on stabilizing a tall wooden structure (just a fun/simple project)

[anduril]

The structure in question is a diving (read: jumping) platform that my father and I built. Here are some pics:

All measurements/approximations were made in feet or inches, so I'll present them in that form if it's not too much of a problem:

The utility poles used are 35ft long, and buried approximately 5ft in the ground. Each pole has a diameter of 10-12 in. at its base, and each pole becomes thinner towards the top. The bases of the poles form an 8ft by 6ft rectangle.

The two decks are situated at about 10ft and 20ft above the ground, and are 16ft long and 8ft wide. Both decks hang out almost 10ft beyond the rectangle of utility poles. The decks are fairly rigid, and are securely attached to the poles with 8 in. lag screws.

Now for the problem: the structure is prone to unwanted vibrations, which are especially apparent when standing on the 20ft platform (on the 10ft one, it isn't bad at all). Before my dad and I resort to reinforcing the thing willy nilly, I thought I would seek out some more educated opinions :)

So far, we have considered adding various wood or metal trusses and/or wire cables anchored to the bases of nearby trees, but we don't know what designs would be most appropriate. We have also considered cutting off a few feet from the tops of the poles to see if that helps, but I thought we might end up putting some trusses up there, so we've held off on that for now.

Anyway, any suggestions would be greatly appreciated.

 

[FGuapo]

First of all, congrats for the whole thing, very engineous. As you said, one great thing you can do is anchor the top of the poles to the ground (or a large tree) very firmly. As well as link the poles to one another so that they're always roughly the same distance a part. 6 meters is quite high, so you have to supress any oscillation of the poles.

 

[archis]

As far as i see you havnt any vertical bonds (or how do you call them in english) in your structure. It might be pritty unstable for horizontal loads and might swing from one side to another

 

[FredGarvin]

That is pretty cool.

Basically, what I would look at doing is trying to "close the structure" to tie in a few of the loose corners that are there. The first thing I would try that would be less invasive would be to tie together the tops of the poles. That should add some torsional stability to the frame.

You will need some form of cross bracing for the horizontal loads though like has already been mentioned.

 

[xxChrisxx]

As archis said, the simplest method is to simply triangualte (get as close to a truss as possible) as much as possible to increase torsional rigidity.

 

[nvn]

I agree with the post by archis. Nice diagram, archis. The diagonal members in the diagram by archis are called diagonals, diagonal members, or diagonal braces. I would currently recommend strong diagonal braces, as shown by archis, not guy wires (at this time). A guy wire acts only in "one half" of one direction (tension, but not compression) on only one pole, but will not tie your structure together to act as a unit internally, whereas strong diagonal braces will (or might), provided the diagonal brace joints are very stiff. Don't cut off the pole tops; that will not help.

I will say, diagonal braces can be cables or rods, if supplied in pairs in an X pattern, called cross bracing. However, keep in mind, stranded cable stretches.